You can help the project in various ways, the best thing to do first is to
subscribe to the mailing-list as explained before, check the archives and the Gnome bug
database:
Provide patches when you find problems.
Provide the diffs when you port libxml2 to a new platform. They may not
be integrated in all cases but help pinpointing portability problems
and
Provide documentation fixes (either as patches to the code comments or
as HTML diffs).
Provide new documentations pieces (translations, examples, etc
...).
Check the TODO file and try to close one of the items.
Take one of the points raised in the archive or the bug database and
provide a fix. Get in touch with me
before to avoid synchronization problems and check that the suggested
fix will fit in nicely :-)